The image density control by AIDC is effected, for example, as follows.
In the first place, a photosensitive surface of a photoreceptor is uniformly charged up to a reference potential Vo by a corona charger controlled for its grid potential to a predetermined potential Vg. Subsequently, a predetermined portion of the photoreceptor surface is subjected to light exposure by a reference light amount Lo, whereby a reference charge latent image (potential Vi) is formed. Then, this reference charge latent image is developed with toner into a visible image by a developing unit controlled for its developing potential to a predetermined potential Vb. Thus, after the test pattern has been formed, the density of said test pattern is detected.
During the actual image formation, depending on the degree of deviation of the above detected density from the density in the normal case (i.e., the density of toner image to be formed under the reference image forming condition), the image forming condition (grid potential Vg, light exposure amount Lo, developing potential Vb, toner replenishing amount, etc.) is set.
More specifically, in the conventional image density control based on AIDC, the test pattern is first prepared, and the image forming conditions have been set so that said test pattern is faithfully reproduced.
Accordingly, when the density of the document image to be reproduced is approximately equal to the density of the test pattern, the document may be faithfully reproduced, but in the case where the density of the document image is different from that of the test pattern, there is such an inconvenience that the document density is not faithfully reproduced.